Epigenetic Memory
What is "Epigenetics"? Epigenetics is a term that used to describe changes that a gene can undergo. This change does not cause a change in the DNA sequence, rather it causes a change to be made "outside" of the gene that regulates that gene in a certain way. There are many different kinds of epigenetic modifications such as DNA methylation and histone modification (aceylation). Plant Memory Plants are exposed to a lot of different elements and conditions. Their environment is always changing and because of that they have developed a defense system called "defense priming" (Kinoshita and Seki 2014). This helps the plant protect itself against herbivores, plant pathogens and environmental stress factors. This paper reviewed different threats to plants and how their gene expression is regulated in each circumstance. Drought Stress Water is a critical part of how plants grow and live. When a plant is in a drought, it needs to conserve water to be able to survive until the next rainstorm. In this study, the researchers looked at the modification of histone tails surrounding drought stress-responsive genes. Histones are an important part of how DNA is packaged within the cell. The DNA strand is tightly coiled up which is then twisted around the histone to make a nucleosome. The nucleosomes are then coiled up tighter into chromatin units and then this forms the chromosome. The histone is one of the areas where regulation can take place. Either acetylation or methylation can occur. Acetylation is the addition of an acetyl group to the DNA wrapped around the histone which causes an increase in that gene transcription. On the other hand, methylation is when a methyl group is added to the DNA on the histone. This causes the DNA to be silenced and there is no transcription of that gene. "Under drought stress conditions,RD29A ,RD20 and AtGOLS2 transcripts accumulate; during rehydration, the amounts of these transcripts fall to the basal level. In concordance with the changing levels of transcripts, acetylation of lysine 9 of histone H3 (H3K9ac), which is correlated with active state of gene expression, and the presence of RNA polymerase II in these genic regions transiently increase under drought stress conditions and then fall during the rehydration recovery period." (Kinoshita and Seki 2014) When the plant was in a normal environment there was no change seen in the transcripts or acetylation of the histones. But as the plant was put under stress, the levels of transcripts increased to compensate for the lack of water. The paper did not indicate what these transcripts were for or what they controlled. Most plants either store their water in their roots, stem, or vacuoles. They have different valves in their stems or roots that can release water to the plant when it needs it. During the periods of re-hydration, the levels of transcripts seemed to fall. This is due to a negative feed back system. If the plant is getting enough water from the environment, then it does not need to conserve water and therefore stops making the transcripts. Where does plant memory (epigenetic memory) fit into all of this? "Plant memory" refers to the face that when plants are exposed to the same stress multiple times in a row, they are better prepared for it the next time around. For example with a drought, if the plant survives the first drought, it is better suited to go through another drought and survive, This epigenetic change seems very small and inconsequential, but this is very important to scientists because this could mean that not everything is determined by genetics. The epigenome could be able to be passed down to the parent's offspring. Another study found that this was true in ''C. elegans. ''Gaydos et. al studied the different epigenetic tags that are passed down on the X chromosome. For both C. elegans sexes, "silenced X chromosomes are a characteristic feature of germline identity and development". In this study they focused on histone H3 lysine 27 trimethylation (H3K27me3) which methylates a certain part of the X chromosome. They noted that the organisms who received a silenced copy from the paternal side were fertile and the ones who received a non-silenced copy did not. It would be advantageous to the organism to inherit and pass on the silenced copy of the X chromosome. References "H3K27 methylation in transgenerational epigenetic memory" Darren J. Burgess, Nature Reviews Genetics 15,703, 2014 doi:10.1038/nrg3848 PubMed id 25324004 "Epigenetic Memory for Stress Response and Adaptation in plants", Kinoshita T and Seki M, Plant Cell Phyioslogy Oct. 8, 2014. PubMed id 25298421